ABSTRACT Cartilage covers the joint surface and absorbs and transmits mechanical forces during normal activity. Given its poor healing capacity and the high incidence of cartilage disease including osteoarthritis, there exists a growing and urgent demand for cell-based repair strategies. Our group has made considerable progress in the creation of 3D synthetic microenvironments in which cells can deposit cartilage extracellular matrix (ECM) and mature into functional engineered cartilage. These studies have employed adult bone marrow derived mesenchymal stem cells (MSCs) coupled with hydrogels based on cartilage ECM (hyaluronic acid (HA)). Despite our progress in this area, several impediments remain in the clinical use of these constructs. These include the finding that MSCs fail to produce engineered tissues that match native properties, fail to fully adopt the chondrocyte phenotype, and are furthermore susceptible to unwanted phenotypic transitions in vivo (i.e., progress to a bone-like phenotype). However, recent studies by our team have identified a novel cohort of progenitor cells that are responsible for synovial joint formation in the embryo, are distinct from progenitors that form bone, and persist within articular cartilage as a progenitor pool. We hypothesize that these synovial joint- forming progenitor cells have a superior and unique capacity to generate cartilage and can be used to engineer and regenerate cartilage with native joint-associated properties. To test this hypothesis, we will develop new methods for cell isolation and expansion, will evaluate differentiation and stability of this cell population (relative to MSCs) in 3D culture, and will tet their translational efficacy in forming functional cartilage in vivo in a large animal defect model In this application, we explore the translational potential of synovial joint progenitor cells throgh two Aims. If successful, this work will develop a unique synovial progenitor cell source and validate the efficacy of this cell type in long-term translational studies. This work has the potential to alter clinical practice by improving patient outcomes after cartilage repair surgeries